The present invention relates to a method for monitoring the tension of a yarn fed to a textile machine and for regulating the feed of the yarn in such a manner as to maintain a constant feed, i.e., equal to a predetermined tension. The yarn is unwound from a usual support, such as a bobbin or the like, and is fed to the machine, such as a conventional loom, a hosiery machine or a knitting machine.
The present invention also relates to a device for implementing the method.
Methods and devices are already known for varying the tension of yarn fed to a textile machine in accordance with particular processing stages of the machine or particular requirements of the user.
A conventional tensioning device typically comprises two opposing disc elements which are pressed together and against an interposed yarn by a helical spring; the action of the spring on the discs is adjusted by a knob which is screwed onto a threaded rod to vary the pressure exerted by the discs on the yarn. In this manner, the yarn is braked and its tension varied.
Another known device comprises a series of fixed pins arranged in such a manner as to compel the yarn to travel over them and undergo a zig-zag path enabling the yarn to be braked or tensioned, By varying the relative spatial position of the pins, the path angles can be varied discretely, thereby adjusting the tension applied to the yarn.
These devices and their adjustment methods, together with other known devices and methods, enable the yarn to undergo the required tensioning or braking, but do not enable this tension to be maintained constant with time.
This tension varies during the use of the yarn even if it originates from a single support member or bobbin. In this respect, the yarn of each bobbin has a starting tension or braking which varies depending on the yarn color, its type, its lubrication and the relative humidity of the environment in which the yarn is processed, but in particular on the bobbin diameter. The yarn bobbin diameter is known to gradually decrease as the yarn is unwound from the bobbin. Hence, the rotational speed under which the yarn is unwound increases in a manner inversely proportional to the bobbin diameter, thus increasing the yarn tension. In addition, if the speed of the textile machine increases there is a proportional increase in the tension applied to the yarn.
The aforesaid conventional devices are unable to independently compensate for the continuous variation in the tension of the yarn during its use in the production of an article.
All this results in various processing problems in textile machines, which affect product quality in addition to resulting in a wastage of yarn used in producing the article. In particular, in knitting and hosiery machines, because of the non-uniformity of the yarn tension, articles are produced without a constant length or width. This makes it difficult to maintain a given size to be produced. In addition, in the case of hosiery this makes it necessary to carry out a final control of the product (matching) consisting of grouping together stockings of a fairly similar length to form suitable pairs. This results in evident high production costs to be added to the excessive use of yarn. In this respect, an article longer or wider than required obviously uses a wasted quantity of additional yarn.
The aforesaid known devices and methods are also unable to independently and automatically compensate for the tension variation. Consequently, in an attempt to maintain yarn tension variations within acceptable limits, operators or users of textile machines are compelled to continuously control the yarn fed to these machines in the sense of continuously measuring its tension and acting on braking devices to control them in such a manner so as to return the tension to within a range of acceptable values.